After beating the Sirens, a giant fish monster and a huge eagle on a frozen ancient land, Sinbad's ship reaches the gates to Tartarus - and Marina has fallen in love with him by now.
They sail off the end of the world - and twist the sails to form a flying machine. Sinbad and Marina then grab a rope and swing through the floating gates to meet with Eris.
So Tartarus is a land of chaos with sand everywhere and a Terracotta army. Eris makes a deal with Sinbad - if he tells the truth she'll give him back the Book of Peace. Sinbad answers her question - that he would rather return to Syracuse, and die, than sail off into the sunset with Marina. Eris says he lied, and they are thrown out of Tartarus.
They get back on the ship and head back to Syracuse to face the music.
Proteus's head is on the chopping block, but Sinbad gets back just in time - and takes his place. The sword falls down - but then shatters. Eris shows up, pretty angry, and as he proved her wrong, she hands him the book and then disappears.
Sinbad turns on the lights again, but isn't one for parties so gets his boat ready.
Proteus figures Marina is about to dump him, so lets her go off with Sinbad. She gets on his ship, and they plan their next adventure together.